CHAPTER SEVEN INTRODUCTORY WELL HYDROLOGY

1. CHAPTER SEVEN INTRODUCTORY WELL HYDROLOGY

2. GROUNDWATER OCCURRENCE

3. ZONES OF AERATION AND SATURATION • In the zone of saturation, all interstices or pores are filled with water under hydrostatic pressure. • The zone of aeration consists of interstices occupied partly by air and water. • The capillary zone has variable thickness and separates the zones of aeration and saturation. • This zone extends from the water table (top of the groundwater) to the limit of capillary rise. • We are interested in the zone of saturation below the water table.

4. SOME HYDRO-GEOLOGIC TERMS • a) Aquifer: A geologic formation or stratum containing water in its voids or pores that may be extracted economically and used as a source of water supply. • An aquifer may be confined or unconfined (see diagram).

5. AQUIFER

6. AQUIFER CONTD. • b) Confined Aquifer: One in which groundwater is confined under pressure greater than atmospheric by overlying impermeable strata. It is also known as artesian or pressure aquifer. • c) Unconfined Aquifer: One in which a water table serves as the upper surface of the zone of saturation. It is also known as a free, phreatic or non-artesian aquifer.

7. HYDRO-GEOLOGIC TERMS CONTD • d) Flowing Artesian Well: Exists when the piezometric surface lies above the ground surface. • e) Aquiclude: A geologic formation so impervious that for all practical purposes it completely obstructs the flow of groundwater (although it may itself be saturated with water). • f) Aquitard: A geologic formation of rather impervious and semi-confined nature which transmits water at a very slow rate compared to an aquifer.

8. HYDRO-GEOLOGIC TERMS CONTD • g) Spring: Abrupt intersection of the ground by the groundwater table. Unlike swamps, the point of intersection is usually followed by a steep slope causing the water to flow by gravity. • h) Ephemeral Stream: Flows seasonally in response to groundwater fluctuations. • i) Intermittent Stream: Flows only after a rain and is fed totally by surface water.

9. HYDRO-GEOLOGIC TERMS CONTD • j) Porosity: Groundwater occurs in voids, or pores of geologic formations. Porosity term is defined as : = W/V where W is the volume of water required to saturate all voids or volume of all pores and V is the total volume of the rock (soil).

10. HYDRO-GEOLOGIC TERMS CONTD • k) Specific Retention (Sr): When groundwater is pumped or drained, some water is retained by molecular and surface tension forces. Specific retention is therefore expressed as: Sr = Wr/V (usually expressed in %). • Wr is the volume of water retained and V is the total volume of rock (soil).

11. HYDRO-GEOLOGIC TERMS CONTD • l) Specific Yield (Sy): The water removed by the force of gravity is the specific yield or effective porosity. It is defined as: Sy = Wy/V (usually expressed in %). • Wy is the volume of water drained while V is the total volume of rock(soil). • Since total volume, W = Wy + Wr • Porosity, : = Sr + Sy.

12. HYDRO-GEOLOGIC TERMS CONTD • m) Storage Coefficient (Storativity, S): It is the volume of water which an aquifer releases from or takes into storage per unit surface area per unit change in head. • Units = L 3 = [ ] ie. dimensionless ie. has no units. • (L2 ) (L) • For the unconfined aquifer, S = Sr

13. HYDRO-GEOLOGIC TERMS CONTD • n) Transmissivity (T): It is a measure of the ease of water movement in the soil. • It is the hydraulic conductivity (K) multiplied by aquifer thickness (b). • ie. T = K x b • m 2 /d m/d x m

14. GROUNDWATER HYDRAULICS • There are two considerations: • a) Steady or unsteady flow conditions • (b) Confined and unconfined aquifers • These flows are different and considered separately. Two basic equations are used in groundwater hydraulics: • i) Laminar flow in porous media - Use Darcy's law. • ii) Conservation of mass - Continuity(Water cannot be created or destroyed)

15. Darcy’s Law G/W Table h1 h2 dl Q = - K A dh/dl

16. Darcy’s Law Contd. • dh/dl is hydraulic gradient in the direction of decreasing head. • The negative means that flow is in the direction of the decreasing head. • Darcy's law is applicable only to laminar flow i.e non-turbulent flow.

17. Reynold's Number • where V is mean velocity, de is effective diameter (90% is larger than). is kinematic viscosity = 1.14 mm2 /s at 15 ° C • When Re is less than 2000, flow is laminar and when it is more than 2000, flow is turbulent. • All groundwater flows are laminar except: • a) Fissured rocks: limestones (b) As water approaches wall of well.

18. Steady Radial Flow to a Well

19. Steady Flow to a Well Contd. • s1 and s2 are drawdowns. • This is the difference in level between the static water level and the pumping water level at a specified point. • Assumptions: (i) There is radial symmetry in wells. • (ii) If any annulus is considered, the total quantity of water is same with other annuli i.e. for continuity, the flow across any annulus is constant. As water approaches a well, the flow rate increases.

20. Assumptions of Derivation Contd. • iii) The groundwater flow is horizontal • (iv) The flow is steady, the drawdown is not increased further.

21. Derivation of Steady State Equation

22. Derivation of Steady State Equation Contd. • Note: The replacement of the head, h with drawdown, s is because the drawdown rather than the head is normally measured during pumping tests. • Given s1 and s2 (drawdowns) at r1 and r2 , and knowing pumping rate, Q, Transmissivity, T can be derived